Method for making a non-linear inductor

ABSTRACT

Non-linear inductor(s) are used to reduce the percent total harmonic distortion of the harmonics in the line currents in the input side rectifier system of an ac drive system. Several constructions for the non-linear inductor(s) are described. The non-linear inductor(s) may be constructed from E and I laminations. The gap depends on the construction of the middle leg of the E laminations and may have a step with a constant spacing or a variable spacing which depends on the stacking of the laminations. Alternatively the non-linear inductor(s) may be constructed from a toriodal core that either has a step gap or a variable type gap.

This is a division of application Ser. No. 10/241,200 filed Sep. 11,2002, now U.S. Pat. No. 6,774,758.

1. Field of the Invention

This invention relates to the input side rectifier in an ac drive systemand more particularly to the inductors used therein.

2. Description of the Prior Art

The two main subsystems of a modern ac drive system are the input siderectifier system and output inverter system. The purpose of therectifier system is to convert input ac voltage, from the utilitysource, into an intermediate dc voltage and the purpose of the invertersystem is to convert the intermediate dc voltage into a variablefrequency and a variable magnitude ac output voltage. The rectifiersystems are also used in equipment such as welding, electroplating anduninterruptible power supplies.

The input rectifier system consists of a three-phase diode bridge,either ac or dc side inductor(s) and dc bus capacitors. The three-phasediode bridge converts input ac voltage into dc voltage. The inductor(s)and capacitor(s) serve as a smoothing filter for the intermediate dcvoltage. Such a rectifier system, when connected to a sinusoidal voltageutility source, draws non-sinusoidal currents. These harmonic currentsare not desirable because of their adverse effects (such as energylosses and malfunction of the sensitive equipment) on the utilitynetwork. Therefore, it is of commercial importance to reduce theharmonic currents produced by the rectifier systems.

The magnitudes of the harmonic currents are mainly dependent on thevalue of the ac or dc side inductors and on the average value of theload current on the dc side. Generally speaking, the level of theharmonic line currents commonly measured in percent total harmonicdistortion (% THD) is lower if the value of the inductor is large. Butthe larger the value of the inductor, the bigger it is in size and themore expensive it is. Also the % THD increases as the load on therectifier circuit is reduced from full load to partial load. Since acdrives operate at partial load for most of their operating time, it isimportant to minimize the % THD of a rectifier circuit at partial load.

The present invention reduces the % THD of the line current of arectifier circuit by incorporating one or more non-linear inductor(s) inthe ac or dc side of the rectifier circuits. Specifically, the inventionreduces the % THD of the line current at partial loads when comparedwith rectifier circuits using conventional (linear) inductors. As anadditional benefit, the invention also reduces current ripple stress onthe filter capacitor at partial loads.

SUMMARY OF THE INVENTION

A method for making a non-linear inductor from magnetic material. Themethod is providing an air gap in the magnetic material that has two ormore widths; and adjusting either each of the two or more air gap widthsor the width of the magnetic material adjacent to the air gap to producea desired non-linear inductance characteristic for the inductor.

A method for making a non-linear inductor from laminations of magneticmaterial having different widths.

The method is stacking the laminations to produce an air gap with two ormore widths; and adjusting the width of the air gap and the number ofthe laminations to produce a desired non-linear inductancecharacteristic for the inductor.

A method for making a non-linear inductor from laminations of magneticmaterial having different widths.

The method is stacking at least a predetermined number of thelaminations having one of the different widths and a predeterminednumber of the laminations having another of the different widths toproduce an air gap with two or more widths; and adjusting thearrangement of the predetermined number of the laminations having one ofthe different widths and the predetermined number of the laminationshaving the another of the different widths in the stack to produce adesired non-linear inductance characteristic for the inductor.

DESCRIPTION OF THE DRAWING

FIG. 1 shows a schematic for a three phase rectifier system that has adc side filter inductor.

FIG. 2 shows a schematic for a three phase rectifier system that has anac side filter inductor.

FIG. 3 shows the current through the inductor of the rectifier system ofFIG. 1 and FIG. 4 shows the line current through phase A of that systemwith a linear inductor.

FIG. 5 shows the current through the inductor of the rectifier system ofFIG. 1 and FIG. 6 shows the line current through phase A of that systemwith a linear inductor and the load on the system at only apredetermined percentage of the rated load.

FIG. 7 shows inductance versus operating current curves for the linearand non-linear inductors.

FIG. 8 shows the current through the inductor of the rectifier system ofFIG. 1 and FIG. 9 shows the line current through phase A of that systemwith a non-linear inductor.

FIG. 10 shows the current through the inductor of the rectifier systemof FIG. 1 and FIG. 11 shows the line current through phase A of thatsystem with a non-linear inductor and the load on the system at only apredetermined percentage of the rated load.

FIG. 12 shows one of the most commonly used construction techniques usedto make a linear dc side inductor.

FIG. 13 shows a construction technique for making a non-linear inductorusing E and I laminations.

FIG. 14 shows another construction technique for making a non-linearinductor using E and I laminations.

FIG. 15 shows a construction technique for making a dc side non-linearinductor in the form of a toroidal core.

FIG. 16 shows another construction technique for making a dc sidenon-linear inductor in the form of a toroidal core.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Referring now to FIG. 1, there is shown a three phase rectifier system10 that has a dc side filter inductor 12 labeled as L. As is shown inFIG. 1, the system has bridge 14 connected directly to the three phases16 a, 16 b and 16 c of the ac input 16. Bridge 14 has six diodes D1 toD6. Input phase 16 a is connected to diodes D1 and D2 at junction 14 a,input phase 16 b is connected to diodes D3 and D4 at junction 14 b, andinput phase 16 c is connected to diodes D5 and D6 at junction 14 c.Filter inductor 12 is connected between the cathode of each of diodesD1, D3 and D5 and one terminal of a capacitor 18, labeled as C, which isin parallel with load 20 represented in FIG. 1 by a resistor labeledRload. The other terminal of capacitor 18 is connected to the anode ofeach of diodes D2, D4 and D6.

FIG. 2 shows a three phase rectifier system 22 in which bridge 14 isconnected to the three phases of ac input 16 through three filterinductors 24 a, 24 b and 24 c. Except for the difference in the locationof the filter inductors, systems 10 and 22 are otherwise identical andtherefore elements in FIG. 2 which have the same function as acorresponding element in FIG. 1 have the same reference numeral that isused in FIG. 1 for that element.

While the present invention is described below in connection with therectifier system of FIG. 1, the description is equally applicable to therectifier system of FIG. 2.

Referring now to FIGS. 3 and 4 there is shown in FIG. 3 the current, IL,through inductor 12 and in FIG. 4 the line current Ia of input phase 16a for a rectifier circuit that has a linear inductor. The inductance ofa linear inductor is substantially constant as a function of the currentflowing through it. As an example, the circuit component values are asfollows:

Vin=400V rms at 50 Hz, line-to-line utility voltage

L=1100 μH

C=1150 μF

Rload=14Ω.

Table 1 below shows the harmonic content of the line current Ia inabsolute values and as a percentage of the total rms current. Thecircuit is operating at the rated power level.

TABLE 1 Harmonic Harmonic Current % of RMS Order (A) Current Fundamental42.66 91.75 5^(th) 13.86 29.80 7^(th) 10.49 22.56 11^(th) 3.84 8.2713^(th) 3.68 7.92 17^(th) 2.49 5.36 19^(th) 2.34 5.03 RMS Current 46.50A (A) % THD 39.78%

Referring now to FIGS. 5 and 6 there is shown in FIG. 5 the current ILthrough the inductor 12 and in FIG. 6 the line current Ia of phase 16 afor a rectifier circuit with a linear inductor but the load on therectifier circuit is at 33% of the rated load, that is, Rload=42Ω. Table2 below shows the harmonic content of the line current Ia in absolutevalues and as a percentage of the total rms current. The rectifiercircuit is operating at a 33% power level.

TABLE 2 Harmonic Harmonic Current % of RMS Order (A) Current Fundamental14.69 76.12 5^(th) 10.01 51.84 7^(th) 7.10 36.81 11^(th) 1.73 8.9713^(th) 1.35 6.98 17^(th) 0.86 4.45 19^(th) 0.74 3.85 RMS Current 19.30A (A) % THD 64.85%

A comparison of the harmonic data from Table 1 and Table 2 shows thatwhen a linear inductor is used for inductor 12, there is a substantialincrease in the percentage of harmonic currents at partial load whencompared with the data at rated load.

Replacing the linear inductor by a nonlinear inductor can substantiallyreduce the harmonic current content of the line current of the rectifiercircuit. The nonlinear (also called swinging choke) inductor has ahigher value of inductance at lower currents but a lower value ofinductance at higher current levels. FIG. 7 shows inductance versusoperating current curves for the linear inductor and for the nonlinearinductor where the amount of core material (laminations) and the numberof turns of the winding are identical in both inductors. Theconstruction method for the nonlinear inductor is described below.

FIGS. 8 and 9 show the inductor current IL and line current Ia of inputphase 16 a at rated load, respectively, with a dc side non-linearinductor. FIGS. 10 and 11 show the inductor current IL and line currentIa of input phase 16 a at 33% load, respectively, with a dc sidenon-linear inductor. Tables 3 and 4, below, show the harmonic currentdata at rated load and at 33% load respectively.

TABLE 3 Harmonic Harmonic Current % of RMS Order (A) Current Fundamental42.51 94.16 5^(th) 11.11 24.60 7^(th) 8.27 18.31 11^(th) 4.47 9.9013^(th) 2.99 6.62 17^(th) 2.48 5.48 19^(th) 2.09 4.64 RMS Current 45.15A (A) % THD 33.67%

TABLE 4 Harmonic Harmonic Current % of RMS Order (A) Current Fundamental14.88 88.86 5^(th) 5.83 34.81 7^(th) 4.51 26.94 11^(th) 1.23 7.3613^(th) 1.36 8.14 17^(th) 0.81 4.81 19^(th) 0.80 4.78 RMS Current 16.74A (A) % THD 45.87%

A comparison of the harmonic data from Table 1 and Table 3 shows thatwhen a non-linear inductor is used, the line harmonics at rated currentare lower than the harmonic currents produced by the linear inductor. Acomparison of harmonic current data from Table 2 and Table 4 shows thatat partial load the non-linear inductor produces a substantially lowerpercentage harmonic currents than the linear inductor.

Referring now to FIG. 12, there is shown the most commonly usedconstruction method to manufacture a linear dc side inductor. Thismethod uses a stack of E and I type magnetic material laminations 30 and32, respectively, and a winding 34 around the middle leg 30 c of the Elaminations. A constant width air gap g1 is introduced at the middle leg30 c of the E laminations. It is also possible to introduce an air gapin the two outer legs 30 a, 30 b of the E laminations. An approximateequation for the inductance value L of this type of the inductor is:$\begin{matrix}{L \propto {\mu_{0}\frac{N^{2}}{\left( {\frac{g1}{1} + \frac{gm}{\mu_{r}}} \right)}}} & {{Eq}.\quad 1}\end{matrix}$where:g1=air gapgm=magnetic path length in the laminationsμ₀=permeability of airμ_(r)=relative permeability of lamination materialN=number of turns in the winding.

Since the relative permeability of the lamination material, μ_(r), isquite high (greater than 1000) as compared with the relativepermeability of the air (equal to 1) in the gap g1, the inductance valueis inversely proportional to the width of air gap g1. In this type ofdesign for a linear dc inductor, the value of the inductance is, as isshown in FIG. 7, fairly constant over the intended operating currentrange. The flux density in the laminations 30, 32 is below thesaturation flux density level and the relative permeability of thelamination material μ_(r) is fairly high. At higher current levels, thelaminations 30, 32 start saturating which means μ_(r) of the laminationmaterial starts to rapidly decrease. Therefore, as seen from Eq. 1,inductance value also starts to decrease as shown in FIG. 7.

FIG. 13 shows one of the two construction methods for the non-linearinductor of the present invention. As with the linear inductor of FIG.12, the non-linear inductor of FIG. 13 uses a stack of E and Ilaminations 40 and 42, respectively and a winding 44 around the middleleg 40 c of the E lamination 40.

The linear dc side inductor of FIG. 12 has a constant air gap width g1.Instead of that constant air gap width the air gap of the non-linearinductor of FIG. 13 has a step width g2 (where g2<g1) for a portion ofthe middle leg 40 c of the E lamination 40. All of the E laminations 40used in the construction of the non-linear dc side inductor have anidentically cut step air gap.

The proportion of the width of the middle leg 40 c of the E lamination40 that produces the smaller air gap g2 with lamination 42 can be variedto achieve the desired non-linearity effect. For example, the inductanceversus current curve for the non-linear inductor shown in FIG. 7 wasachieved by choosing the width of gap g2 to be equal to 25% of the widthof gap g1 and the width of the small air gap g2 was 40% of the width ofthe middle leg 40 c of the E lamination 40.

The non-linear behavior of the dc side inductor shown in FIG. 13 can beexplained as follows. At low operating currents, the or of thelaminations 40, 42 is high and the inductance is dominated by the smallair gap g2 and therefore the inductance value is high. As the operatingcurrent increases, the lamination material below the small air gapstarts to saturate and μ_(r) decreases rapidly with the consequent rapiddecrease in inductance that is shown in FIG. 7 for the non-linearinductor.

When such a non-linear inductor is used as the dc side filter inductor12 in the rectifier circuit 10 of FIG. 1, the higher value of theinductance at low operating currents (partial load) produces a lowermagnitude of the current ripple through the inductor, as shown in FIG.10, when compared with the ripple produced by the linear inductor atpartial load as shown in FIG. 5. This reduction in the ripple current isresponsible for the lower harmonic currents due to the non-linearinductor at partial loads, as shown in Table 4, when compared with theharmonic currents produced by the linear inductor at partial loads shownin Table 2.

FIG. 14 shows the second method to construct the non-linear inductor ofthe present invention. In this construction the middle leg 50 c of someof the E laminations 50 have a constant width air gap g1 with lamination52 and the middle leg 50 c of the remainder of the E laminations 50 havea different value of constant width air gap g2 (where g2<g1) withlamination 52. The ratio of the number of laminations 50 with a middleleg 50 c that has a small air gap with lamination 52 to those that havea big air gap with lamination 52 can be chosen to achieve the desirednon-linear effect. The inductance versus current curve for thenon-linear inductor shown in FIG. 7 was achieved by choosing that ratioto be equal to 2:3.

The small and big air gap laminations can be placed in differentpositions relative to each other. In the non-linear inductor of FIG. 14they are shown with those laminations of the middle leg 50 c thatproduces the smaller air gap with laminations 52 in the center of theentire stack. Another option is to reverse the arrangement where the bigair gap laminations are in the center of the stack. Some otherarrangements are: side-by-side positioning of small and big air gaplaminations or dispersing small and big air gap laminations uniformlythroughout the stack of E laminations 50.

While the constructions shown in FIGS. 13 and 14 for the non-linearinductor of the present invention uses a stack of E and I laminationswith the air gap in the middle leg of the E lamination it should beappreciated that:

-   -   a) an air gap having the characteristics described above for the        embodiments of FIGS. 13 and 14 may also be at either of both of        end legs 40 a and 40 b for the embodiment of FIG. 13 and at        either or both of end legs 50 a and 50 b for the embodiment of        FIG. 14; and    -   b) the non-linear inductor in both figures may also be embodied        using for example only E laminations or using U shaped        laminations and I laminations or using only U shaped laminations        or any other shape or combination of shapes of laminations that        allow for one or more gaps that have the characteristics        described above for the embodiments of FIGS. 13 and 14.

FIG. 15 shows a construction method for a dc side non-linear inductor inthe form of a toroidal core 60. In this construction a tape of amagnetic material is wound in a toroidal shape. An air gap 62 isintroduced by cutting core 60 in an axial direction. As shown in theFIG. 15, the larger air gap g1 is placed on the outer edge of thetoroidal core 60 and smaller air gap g2 is placed in the middle of thetoroidal core 60. In this sense, the construction of FIG. 15 is thetoroidal equivalent of the stepped gap E-I construction of FIG. 13.

FIG. 16 shows a construction method for a dc side non-linear inductor inthe form of a toroidal core 70 which as is described below has an airgap that is different in construction than the air gap of toroidal core60 of FIG. 15. In core 70 a tape of a magnetic material is wound in atoroidal shape. An air gap 72 is introduced by cutting core 70 in aradial direction. But instead of a constant air gap, the tape or“laminations” on the outside (diameter) of the toroidal core 70 have abigger air gap (g1) than the air gap (g2) in the tape or “laminations”which are inside (diameter) the toroidal core 70. In this sense, theconstruction of FIG. 16 is the toroidal equivalent of the variable gapE-I construction of FIG. 14.

It is to be understood that the description of the preferredembodiment(s) is (are) intended to be only illustrative, rather thanexhaustive, of the present invention. Those of ordinary skill will beable to make certain additions, deletions, and/or modifications to theembodiment(s) of the disclosed subject matter without departing from thespirit of the invention or its scope, as defined by the appended claims.

1. A method for making a non-linear inductor comprising the steps of:providing a plurality of first laminations of magnetic material, each ofsaid first laminations having a first leg with an edge and at least oneother leg; providing a plurality of second laminations of magneticmaterial, each of said second laminations having the same predeterminedshape; stacking said first laminations to produce a first stack with afirst leg portion and at least one other leg portion, said first legportion comprising the first legs and said at least one other legportion comprising the at least one other legs; stacking said secondlaminations to produce a second stack; disposing the first stackadjacent to the second stack so as to form an air gap with two or moredifferent widths between the first leg portion of the first stack andthe second stack; disposing a winding around the first leg portion orthe at least one other leg portion of the first stack; and adjusting theconfiguration of said air gap to produce a desired non-linear inductancecharacteristic for said inductor; wherein in a first predeterminednumber of said first laminations, the first leg has a first length andin a second predetermined number of said first lamination, the first leahas a second length, said first and second lengths being different; andwherein the step of adjusting the configuration of the air gap comprisesadjusting the arrangement of said first predetermined number of saidfirst laminations having said first length and said second predeterminednumber of said first laminations having said second length in said firststack to produce the desired non-linear inductance characteristic forsaid inductor.
 2. The method of claim 1, wherein the air gap comprises afirst portion having a first width and a second portion having a secondwidth, said first and second widths being different.
 3. The method ofclaim 2, wherein the step of adjusting the configuration of the air gapcomprises adjusting the width of one of the first and second portions ofthe air gap.
 4. The method of claim 2 wherein the step of adjusting theconfiguration of the air gap comprises adjusting a length of one of thefirst and second portions of the air gap.
 5. The method of claim 2,wherein each of the first laminations is E-shaped and includes a centerleg disposed between pair of end legs, said center leg being the firstleg and said end legs being the at least one other leg; and wherein thefirst stack comprises a center leg portion disposed between a pair ofend leg portions, said center leg portion being the first leg portion ofthe first stack and said end leg portions being the at least one otherleg portion of the first stack.
 6. The method of claim 5, wherein thefirst and second portions of the air gap are arranged adjacent to eachother in the stacking direction of the first laminations.
 7. The methodof claim 5, wherein the winding is disposed around the center legportion of the first stack, wherein each of the second laminations isI-shaped, and wherein the first stack is disposed adjacent to the secondstack such that end portions of the second stack adjoin the end legportions of the first stack, respectively.
 8. A method for making anon-linear inductor comprising the steps of: providing a plurality offirst laminations of magnetic material, each of said first laminationshaving a first leg with an edge and at least one other leg; providing aplurality of second laminations of magnetic material, each of saidsecond laminations having the same predetermined shape; stacking saidfirst laminations to produce a first stack with a first leg portion andat least one other leg portion, said first leg portion comprising thefirst legs and said at least one other leg portion comprising the atleast one other legs; stacking said second laminations to produce asecond stack; disposing the first stack adjacent to the second stack soas to form an air gap with two or more different widths between thefirst leg portion of the first stack and the second stack; disposing awinding around the first leg portion or the at least one other legportion of the first stack; and adjusting the configuration of said airgap to produce a desired non-linear inductance characteristic for saidinductor; wherein the air gap comprises a first portion having a firstwidth and a second portion having a second width, said first and secondwidths being different; and wherein the air gap further comprises athird portion having the first width, and wherein said second portion isdisposed between the first and third portions.
 9. The method of claim 8,wherein the second portion of the air gap is located midway along thedepth of the first stack in the stacking direction.
 10. The method ofclaim 8, wherein the first width is greater than the second width. 11.The method of claim 8, wherein the second portion of the air gap extendsthe entire depth of the first stack in the stacking direction.
 12. Themethod of claim 8, wherein each of the first laminations is E-shaped andincludes a center leg disposed between a pair of end legs, said centerleg being the first leg and said end legs being the at least one otherleg; and wherein the first stack comprises a center leg portion disposedbetween a pair of end leg portions, said center leg portion being thefirst leg portion of the first stack and said end leg portions being theat least one other leg portion of the first stack.
 13. The method ofclaim 12, wherein the winding is disposed around the center leg portionof the first stack.
 14. The method of claim 12, wherein each of thesecond laminations is I-shaped, and wherein the first stack is disposedadjacent to the second stack such that end portions of the second stackadjoin the end leg portions of the first stack, respectively.
 15. Themethod of claim 12, wherein the air gap extends uninterrupted betweenthe center leg portion of the first stack and the second stack for theentire width of the center leg portion of the first stack.
 16. A methodfor making a non-linear inductor comprising the steps of: providing aplurality of first laminations of magnetic material, each of said firstlaminations having a first leg with an edge and at least one other leg;providing a plurality of second laminations of magnetic material, eachof said second laminations having the same predetermined shape; stackingsaid first laminations to produce a first stack with a first leg portionand at least one other leg portion, said first leg portion comprisingthe first legs and said at least one other leg portion comprising the atleast one other legs; stacking said second laminations to produce asecond stack; disposing the first stack adjacent to the second stack soas to form an air gap with two or more different widths between thefirst leg portion of the first stack and the second stack; disposing awinding around the first leg portion or the at least one other legportion of the first stack; and adjusting the configuration of said airgap to produce a non-linear inductance characteristic for said inductor;wherein the air gap comprises a first portion having a first width and asecond portion having a second width, said first and second widths beingdifferent; and wherein the first laminations each have the samepredetermined shape, wherein in each of the first laminations, the edgeof the first leg is a stepped edge, and wherein the step of stacking thefirst laminations comprises aligning the stepped edges of the first legsto form a stepped end of the first leg portion of the first stack. 17.The method of claim 16, wherein each of the first laminations isE-shaped and includes a center leg disposed between a pair of end legs,said center leg being the first leg and said end legs being the at leastone other leg; and wherein the first stack comprises a center legportion disposed between a pair of end leg portions, said center legportion being the first leg portion of the first stack and said end legportions being the at least one other leg portion of the first stack.18. The method of claim 17, wherein the first and second portions of theair gap are arranged adjacent to each other in the direction between theend leg portions of the first stack.
 19. The method of claim 17, whereinthe winding is disposed around the center leg portion of the firststack, wherein each of the second laminations is I-shaped, and whereinthe first stack is disposed adjacent to the second stack such that endportions of the second stack adjoin the end leg portions of the firststack, respectively.